In recent years, in an optical communication transmission technology, a request for speed-up, high frequency, high integration, multi-level modulation, or the like has further increased, and also with respect to an optical device which is used in the optical communication transmission technology, an optical device which is applied to attain speed-up, high frequency, high integration, multi-level modulation, or the like has been required.
In the related art, in an optical device, in order to respond to a downsizing according to high frequency or high integration, an optical device having a coplanar type structure in which an upper ground electrode is formed adjacent to a signal electrode on the upper surface of lithium niobate (LiNbO3) or the like having an electro-optic effect has been proposed (Patent Literature No. 3). Hereinafter, the coplanar type structure is referred to as a CPW structure.
In order to further increase modulation efficiency than that in the CPW structure, an optical device having a microstrip structure in which an electro-optical substrate using LiNbO3, resin, or the like is thinned, and a signal electrode and a ground electrode are respectively formed on the upper surface and the lower surface of the substrate, whereby the electro-optical substrate is sandwiched between the signal electrode and the ground electrode in an up-and-down direction, has been proposed (refer to Patent Literature Nos. 1 and 2, and the like). Hereinafter, the microstrip structure is referred to as an MSL structure.
In order to stabilize characteristics in the CPW structure, an optical device in which an upper ground electrode is formed adjacent to a signal electrode on the upper surface of an electro-optical substrate, whereby the CPW structure is made, and a lower ground electrode is formed on the lower surface of the electro-optical substrate, has also been proposed (Patent Literature No. 3). Among such optical devices, an optical device made to be operable at high speed by realizing velocity matching between a microwave and a light wave, or impedance matching of a microwave has also been proposed.
Further, an optical device having a structure made such that an electrode layer is not provided at a cutting position when a wafer in which a plurality of optical devices each having a structure in which an optical waveguide is sandwiched between a pair of electrode layers in an up-and-down direction are formed is cut into individual chips (optical devices) has also been proposed (Patent Literature No. 4).